As is known, in the mobile communication environments the maintenance of a traffic connection with a mobile station is made possible with the help of a handover function. In a handover, which is also called a handoff, a new connection is established with a new coverage area, such as a cell, and the connection with the old coverage area is released, i.e. the mobile station and its traffic connection are handed over to a new cell. Along the convergence of the Internet and wireless communication, mobility support has also been developed for IP networks. IETF (Internet Engineering Task Force) has defined a mechanism for supporting mobility in IP networks. The official name of the mechanism is IP Mobility Support, but it is also called Mobile IP. Mobile IP describes the basic operations needed for a mobile node to maintain its connectivity to the Internet during its handover from one IP access point to another.
Due to the increasing mobility in the networks, the handover mechanisms have to be developed continually. There are two major problems associated with this development process: the data loss which a handover causes if no extra measures are taken and the latency related to the handover. Furthermore, these two problems are interconnected; the faster the handover, the shorter the period during which packets can be lost.
The proposals for eliminating data loss in connection with a handover are mostly based on the buffering of data in one or more routers (access points). One solution is based on multicast transmission, i.e. the same data is transmitted to several routers (access points) surrounding the router currently forming the access point of the node. Thus, the data is already available for the mobile node when it performs a handover.
As a handover involves various operations, such as location update and rerouting of the data, the combined latency can be appreciable, at least for some applications. Thus, there is also a need for fast handovers which reduce this latency. Latency caused by rerouting of the data can be minimized by so-called predictive handover schemes, where the potential access points are predicted and the connection is pre-established to these access points.
The article “An IP Mobility Support Architecture for the 4GW Wireless Infrastructure” by Jian Wu, at the website http://www.s3.kth.se/radio/4GW/public/Papers/JWWS99.pdf (visited in September 2002), describes a method for tackling the latency problem. The paper describes a new network architecture including an additional agent, Mobility Support Agent (MSA), added to the IP subnetworks. Prior to a handover, the mobile node pre-registers itself with the MSA of the relevant neighboring subnetwork. This MSA then negotiates with a designated router or the home agent of the mobile node in order to redirect the traffic so that it is available for the mobile node when the node arrives at the new sub network.
The article “MOMBASA: Mobility Support—A Multicast-based Approach”, by A. Festag and A. Wolisz, available at the website http://www-tkn.ee.tu-berlin.de/publications/papers/festag_ew2000.pdf (visited in September 2002), describes a predictive handover scheme in which the base stations which are potential candidates for the handover of the mobile form a set. The set can be optimized using topological knowledge.
With the rapid increase of wireless services and mobility towards truly mobile computing and networking, with computing devices commonplace in all kinds of vehicles, for example, the ability to provide seamless connections becomes even more important. Furthermore, with diminishing cell sizes in conventional mobile networks and with short-range radio systems and ad-hoc networks becoming more common, the maintenance of seamless connections will be more and more demanding. Therefore, the problem of achieving seamless communications in an efficient way, i.e. without excessive consumption of existing network resources and without a need for additional network elements, will be more and more emphasized.